Well casing perforation technology has developed over a number of years into a rather sophisticated art. Tubing conveyed perforating guns, which are common today, are lowered into a well bore which has been lined with a casing until they are in the area of the bore adjacent the geological zone containing desired oil or gas. A firing head is used with the guns to cause charges in the guns to explode. The explosions result in holes in the casing, allowing the gas or oil in the formation to flow into the casing. A tubing string extending through the casing bore is used to flow the oil or gas to the surface of the earth.
With tubing conveyed perforating, two types of firing heads have become most popular for detonating the perforating gun charges. The first type is a mechanically-actuated device that is actuated by a rod which is dropped down the tubing string from the surface. The rod strikes a "plunger" which, in turn, strikes an initiator, causing the charges to explode. The second type is a pressure-actuated head wherein fluid or gaseous pressure is applied to the tubing string bore. The pressure is transmitted to the firing head "plunger" which is held in place by shear pins. When the pressure reaches a pre-determined level, the pins are sheared and the plunger is forced against the initiator. The charges are thus caused to explode.
Each of these popular techniques has its drawbacks. The rod is susceptible to hanging up in the tubing string and special provisions are required to ensure a clear, straight path to the firing head. The pressure-actuated devices are not particularly well suited for applications where a large underbalance condition is desired or where the formation pressure is low and an underbalanced condition is desired or required. These heads typically require pressures in the range of 4000 psi or greater for actuation, severely limiting the ability to maintain an underbalanced condition for perforation.
Highly deviated wells present additional problems. In many applications, a well bore may deviate from the vertical by as much as 80.degree. and this nearly horizontal deviation may extend for several thousand feet. In a well such as that, the sliding rod actuated firing head is not useable. The pressure-actuated firing head has been used and its disadvantages have been tolerated.